Patent GB-708536-A (GB)
Title: Improvements in or relating to apparatus of the kind for measuring electrical impedance or a characteristic dependent upon electrical impedance
IPC: G01R27/02
Filed: 1951-04-04
Abstract: 708,536. Aerial-feeding arrangements; testing radio frequency impedance. GENERAL ELECTRIC CO., Ltd., BAYLISS, A. J., BLOCH, A., FISHER, F. J., and HUNT, G. J. April 2, 1952 [April 4, 1951], No. 7828/51. Class 40 (5). [Also in Group XXXVI] Apparatus for measuring an impedance or matching an impedance (e.g. of an aerial) to a transmission line comprises a section of transmission line (which may be e.g. a coaxial line, a waveguide, or a slab line comprising a centre conductor located between parallel side plates) connected between a steady source of modulated R.F. and the load impedance to be determined and having three probes spaced along its length to develop three separate signals proportionate to the amplitudes of line oscillations at the appropriate probes, after which the signals are separately and variably attenuated and different pairs of signals are balanced one against the other in centre zero indicating instruments, the value of the terminating impedance of the line section being calculated from the attenuation valves inserted in the three signal channels to produce zero balance indication. In Fig. 3 a modulated R.F. oscillator 1 of e.g. 70-88 mc/s is coupled through a flexible coaxial connection to a rigid coaxial line 3 of impedance, e.g. 75 ohms having three equally (e.g. 19 in.) spaced pick up probes which is terminated in a load impedance 4. Each probe unit 5 comprises, Fig. 4 (not shown) a closely mounted diode demodulator and filter network, and the several signals are connected to an attenuator unit 13, Fig. 5, comprising in each channel two cascaded 600 ohm decade attenuaters 21, 22 operating in 1 and 10 db steps, whose outputs are fed through 8 db pads 23, terminating resistances 24, and 10 1 step up transformers 25 to the amplifier unit 14, Fig. 6. In the latter, the signals are respectively coupled to the inputs of degenerative amplifiers 26, 27, 28 of which 26, 28 have single cathode follower outputs feeding bridge rectifiers 39, 42, and 27 has two parallel cathode follower outputs feeding bridge rectifiers 40, 41. The gains of amplifiers 26, 28 are variable by feedback control potentiometers 34, 35 and the outputs of rectifiers 39, 40 and 40, 41 are combined in opposed pairs and respectively applied to centre-zero meters 44, 45 through ganged tapping switches 46, 47 of universal shunts 48, 49 varying the sensitivity of the meters, whose indications are thus proportionate to the differences between the electric fields in the line at the first and second and second and third probes, respectively. Switches 51, 61 are provided to correct the inputs of the amplifiers and attenuators respectively in parallel to facilitate the equalization of the amplification and attenuation channels by variable resistances 45, 35 and potentiometers 18, 62. A preliminary procedure is described for adjusting the pickup probes and symmetrically matching the three probe line to e.g. 75 ohms impedance, utilising a 75 ohm # section substandard dummy load, Fig. 7 (not shown) and a calibrating section of 75 ohm coaxial line adapted to shift the standing wave pattern along the 3-probe line by the distance between adjacent probes. An unknown impedance is adjustable to e.g. 75 ohms by substituting it for the substandard after calibration, and adjusting for zero indication of the meters with equal attenuation in each channel. An unknown impedance (or the standing wave ratio of maximum to minimum voltage or current in a line of e.g. 75 ohms due to such a load) is determined by connecting the load to the three-probe line output either direct or over a half-wave section of coaxial cable, adjusting the attenuators for zero meter indications, and determining from the attenuations A, B, C in the three probe channels the values B-A and B-C. By means of a calculating chart comprising curves representing B-A and B-C superimposed on a circular polar co-ordinate diagram the polar coordinates p, # are computed to respectively represent the modulus and argument of the reflection coefficient. Formulµ are given connecting the standing wave ratio for a 75 ohm line due to the load, the equivalent series resistance and reactance, and the equivalent parallel resistance and reactance, of the load. A geometrical construction for the calculation chart is given, Figs. 1, 2 (not shown).